sim/ppc: fix for operator precedence warning from clang

In the ppc simulator, clang was warning about some code like this:

  busy_ptr->nr_writebacks = 1 + (PPC_ONE_BIT_SET_P(out_vmask)) ? 1 : 2;

The warning was:

  operator '?:' has lower precedence than '+'; '+' will be evaluated first

I suspect that this is not the original authors intention.
PPC_ONE_BIT_SET_P is going to be 0 or 1, so if we evaluate the '+'
first, the condition will always be non-zero, so true.  The whole
expression could then be simplified to just '1', which doesn't make
much sense.

I suspect the answer the author was expecting was either 2 or 3.  Why
they didn't just write:

  busy_ptr->nr_writebacks = (PPC_ONE_BIT_SET_P(out_vmask)) ? 2 : 3;

I have no clue, however, to keep the structure of the code unchanged,
I've updated things to:

  busy_ptr->nr_writebacks = 1 + (PPC_ONE_BIT_SET_P (out_vmask) ? 1 : 2);

which silences the warning from clang, and is, I am guessing, what the
original author intended.